Electric discharge lamp



511337 30, 1946. v. J. FRANCES ETAL ELECTRIC DISCHARGE LAMP Filed Jan.1, 1945 Invent ofs H e m n 0 mm m. H A & r Hm e n h mm T EV uu BPatented July 30, 1946 ELECTRIC DISCHARGE LAMP Victor J. Francis,Aylesbury, and Evan H. Nelson, Harrow Weald, England, assignors toGeneral Electric Company, a corporation of New York Application January1, 1945, Serial No. 570,892 In England July 2, 1943 2 Claims.

This invention relates to high-intensity gaseous electric dischargelamps, and especially to high pressure metal vapor H. P. M. V.) electriclamps of the type wherein the brightness of the discharge column exceeds10,000 candles per sq. cm. Here and hereinafter brightness means maximumbrightness in a cross-plane that is to say, in a plane perpendicular toand intersecting the shortest straight line between the electrodes.

. In one kind of lamp of this type the length of the envelope and thedistance between the electrodes are much greater than the diameter ofthe envelope in a cross-plane midway between the electrodes. It is thennecessary to cool the envelope with liquid applied to its outer surface.But now the difliculty arises that, if the cooling is suificient toprevent the central part of the envelope from being damaged by heat, itis difficult to prevent the parts behind the electrodes from being socool that the requisite high vapor pressure of the metal (usuallymercury) can not be maintained. It is overcome by providing within theenvelope metal so much in excess of that evaporated in full operationthat the cool spaces behind the electrodes are filled with metal. Thesurface of this metal might actually be the elec trode, but it is usualto provide tungsten electrodes, projecting slightly from the mercury.

These electrodes are kept below their melting point by the conduction ofheat through the mercury between them and the wall. But the presence ofthe excess mercury limits the operating characteristics of the lamp, andthe necessity for keeping it around the electrode limits the form of thelamp.

It has heretofore been proposed to avoid the necessity for excessmercury by bringing solid electrodes into sufficiently close contactwith the end wall of the envelope. It was also stated that this devicemade it possible to make the diameter of the central part of theenvelope greater than was usual in water-cooled H. P. M. V. lamps at thedate; but it was not suggested that the prevailing practice of makingthe distance between the electrodes much greater than the diameter ofthe envelope should be abandoned.

A second kind of high brightness H. P. M. V. lamp is known, in which thedistance between the electrodes is less than the diameter of theenvelope and indeed less than the distance of either electrode from theenvelope; the envelope is approximately spherical and all the mercury isevaporated in normal operation. In such lamps, if a coolant has beenapplied to the exterior cf the envelope during normal operation, it hasbeen a gas (usually air). A forced blast of air may be provided; but itis impossible or at least very inconvenient to make air cooling asefficient as water cooling. Accordingly, for the same power dissipatedwithin the envelope, the area of the external surface of high brightnesslamps of the second kind has to be larger than that of high brightnesslamps of the first kind. Moreover, if the area of the envelope werereduced by water cooling, it would not alwa s be possible to reduce thesize of the electrodes so that they would still fit inside the envelope,for the electrodes are cooled mainly by radiation. For these reasonslamps of the second kind are usually bulkier than corresponding lamps ofthe first kind. But it is not a serious disadvantage so long as thepower dissipated by the lamp is less than 2 kw., for then the diameterof the envelope need not be more than mm. However when the powerdissipated is greater than 5 kw., the disadvantage is serious; an objectof the invention is to remove it.

We have discovered that it can be removed by using as an electrode astout rod of refractory metal, on whose inner end the dischargeterminates and whose outer part fits closely into a tube projecting fromthe main, approximately spherical, envelope. The whole envelope can thenbe water cooled. The conduction down the rod to the part within thetube, through which much of the heat developed at the electrode isabstracted, is suiiicient to prevent the inner end from melting, and yetthe space between the rod and the tube is kept so hot by the rod thatsubstantially all the metal is evaporated in normal operation. If thelamp is run on A. C., both electrodes must be of this kind; but if, asis now usual with high brightness lamps, the lamp is run on D. 0., itmay be possible to use as a cathode an electrode of some other kind.

According to one aspect of the invention, in a high brightness H. P. M.V. electric discharge lamp, the envelope consists of a main,approximately spherical, part of external area S, having at least onetubular neck portion projecting therefrom; one electrode is a stout rodof refractory metal on whose inner end the discharge terminates andwhose outer part fits closely within the said tube; the distance betweenthe terminations of the discharge in normal operation is less than thediameter of the envelope in any crossplane; and the lamp is adapted,when the whole exterior f the envelope is water-cooled, to dissipate apower W, where W is not less than 4 kw. and W/S is not less than 60watts/sq. cm.

According to another aspect of the invention, a source of light, adaptedto cooperate with optical projection apparatus, comprises in combinationa H. P. M. V. lamp according to the first aspect of the invention andmeans for maintaining liquid in contact with substantially the whole ofthe surface of the said envelope. Preferably the said means is a jacket,of the same material as the envelope and forming part of the samevitreous body, provided with apertures for the entry and exit of theliquid.

One embodiment of a lamp comprising the invention will now be described,by way of example, with reference to the accompanying drawing which isan elevation, in section, of such a lamp.

Here I is the main part of the envelope, being a sphere 40 mm. inexternal diameter with a quartz wall 2-3 mm. thick. 2 and 3 are tubesforming tubular neck portions projecting from it at opposite ends of adiameter, the thickness of their quartz walls being 1 -1 mm. The anode4, is a tungsten cylinder, 40 mm. long and 12 mm. in diameter, fittingclosely within tube 2 and with its inner end projecting 2-3 mm. into thesphere I; its outer end is connected to exterior leads through themultiple strip seal 5. The seal 5 comprises a plurality of thin metallicstrips 6 of molybdenum, for example, attached to the outer end of theelectrode 4 and fused between an extension I of tube 2 and a quartz plug8. The cathode 9 is a tungsten rod, 50 mm. long and 6 mm. in diameter,fitting closely within the tube 3 and with its inner end projecting sofar within the sphere I that it is 10 mm. distance from the inner end ofthe anode. Near its inner end it carries the activated startingelectrode I0; its outer end is connected to exterior leads by the lowerstrip seal 5. I I is a quartz jacket surrounding the whole of the partsvI, 2, 3, and sealed at its ends to the outside of the seals 5, 5. Thereis 3-6 mm. clearance between the jacket and the parts I, 2, 3. I2, I3are respectively inlet and outlet tubes for water passing through thejacket.

The lamp is designed to burn on a D. C. supply with the axis verticaland the cathode lowermost. The amount of mercury, practically allevaporated in operation, is adjusted so that, when the discharge is inseries with a suitable stabilizing resistance, the current carried bythe discharge is 77 amps. and the voltage between the electrodes 65volts. The brightness of the discharge exceeds 20,000 candles per sq.cm.; the power dissipated is 5 kw.

What we claim as new and desire to secure by Letters Patent of theUnited States is:

1. A high-pressure electric discharge lamp comprising a globularenvelope of vitreous material and designed for operation under liquidcooling conditions with a power input in excess of watts per squarecentimeter of the external surface of said envelope and a total powerinput in excess of 4 kw., a predetermined amount of mercury in saidenvelope which is completely vaporized during operation of the lamp, athin-walled elongated tubular neck portion projecting from saidenvelope, an electrode in the form of an elongated stout rod ofrefractory metal which fits closely within but is not hermeticallysealed to said neck portion and terminates, at its inner end, close tothe inner wall of the envelope, current conductor means connected to theouter end of said electrode and hermetically sealed in the outer end ofsaid envelope neck portion, a cooperating electrode sealed in theenvelope at a point opposite the first-mentioned electrode, the arc gapbetween said electrodes being shorter than the diameter of the envelopein any plane normal to the shortest straight line between saidelectrodes.

2. A high-pressure electric discharge lamp comprising a globularenvelope of vitreous material and designed for operation under liquidcooling conditions with a power input in excess of 60 watts per squarecentimeter of the external surface of said envelope and a total powerinput in excess of 4 kw., a predetermined amount of mercury in saidenvelope which is complete vaporized during operation or" the lamp, apair of thinwalled elongated tubular neck portions projecting fromopposite sides of said envelope, a pair of electrodes in the form ofelongated stout rods of refractory metal which fit closely within butare not hermetically sealed to said neck portions, current conductormeans connected to the outer ends of said electrodes and hermeticallysealed in the outer ends of said envelope neck portions, the arc gapbetween said electrodes being shorter than the diameter of the envelopein any plane normal to the shortest straight line between saidelectrodes.

VICTOR J. FRANCIS. EVAN H. NELSON.

